This invention relates to an ion-exchange fiber with practical fiber properties, obtained from a polyvinyl alcohol type fiber (hereinafter referred to as PVA type fiber) as the starting material.
Ion-exchange resins are usually produced by introducing an ion-exchange radical into a support of organic high molecular materials, for example, a styrene/divinylbenzene copolymer in the form of particles or planes by a chemical reaction.
If the ion-exchange radical is introduced into a fibrous support of organic high molecular materials, an ion-exchange fiber having the advantages of fibers can be obtained. However, when ion-exchange fibers having the same exchange capacity as one of the ion-exchange resin are dipped in water, they swell remarkably because of the ion-exchange radical being hydrophilic, so that the fiber cannot be used in practice due to the lowering of the physical properties. Therefore, the exchange capacity to be rendered to fibers is 2.5 mg eq./g at the most as a salt-splitting capacity.
Recently, carbon fibers have been developed to practical uses and as one of the applications, an ion-exchange fiber of heat and chemical resistances, in which carboxyl radical was introduced by oxidation or sulfonic radical was introduced by sulfonation has been provided. For example, Japanese Patent Publication No. 7523/72 discloses a process for the production of an ion-exchange fiber by introducing ion-exchange radicals into an electrically conductive carbon fiber. Also, a process for introducing ion-exchange radicals into carbon fibers starting from cellulose fibers or polyacrylonitrile fibers is disclosed in Japanese Patent Publication No. 37680/72.
However, ion-exchange fibers having a salt-splitting capacity of more than 2.5 mg eq./g cannot be obtained even if carbon fibers are reacted with sulfuric acid under drastic conditions to render them a strongly acidic cation exchange capacity. Thus, the exchange capacity of the ion-exchange fibers is remarkably inferior to that of the ion-exchange resins. The term "salt-splitting capacity" used herein is defined as follows: A column is filled with a strongly acidic cation exchanger in the form of H type and a solution of neutral salts such as sodium chloride is passed through. The salt-splitting capacity is indicated by equivalent numbers of acid formed to the weight of a sample.